In the ophthalmologic field, various kinds of stereomicroscopes such as a slit lamp microscope and a surgical microscope are used. An ophthalmologic stereomicroscope is generally configured to enable binocular observation of an eye. Moreover, such a stereomicroscope is configured to enable observation of not only an anterior segment (the cornea and so on) but also the inside of an eye. The “inside of an eye” refers to a site within the pupil, for example, the fundus oculi (the retina) and the vitreous body.
In intraocular observation using a stereomicroscope, a specific lens is placed between an objective lens and an eye. For example, in a slit lamp microscope described in Patent Document 1, a head lens placed in non-contact with an eye is used. Although the head lens is held by a lens holder in Patent Document 1, the head lens may be grasped by the operator and used.
A surgical microscope described in Patent Document 2 is configured to enable intraocular observation in a state that a head lens is placed in non-contact with an eye. Moreover, Patent Document 3 discloses a contact lens placed in contact with an eye during surgery.
In this description, a lens placed between an objective lens and an eye for intraocular observation as described above will be generically referred to as a head lens.    [PATENT DOCUMENT 1]    Japanese Unexamined Patent Application Publication No. 9-276227    [PATENT DOCUMENT 2]    Japanese Unexamined Patent Application Publication No. 2005-230558    [PATENT DOCUMENT 3]    Japanese Unexamined Patent Application Publication No. 2003-24366